The invention relates to a method for continuously drying boards coated on both sides which are conveyed by conveyor rollers horizontally through the drying zone, and are heated on the top side and cooled on the bottom side, and also to an apparatus for continuously drying boards coated on both sides.
To produce conductor track patterns on printed circuit boards by the photographic printing method, copper-laminated insulating material is first provided with a photosensitive layer, the etching resist or electroplating resist. In a further step, the structured conductor track patterns may be coated with photosensitive solder resists which, on the one hand, protect the unbared conductor tracks against the liquid solder in the soldering process and, on the other hand, protect them against corrosion when loaded with current, in particular at high atmospheric humidity.
Said photosensitive layers are deposited either as dry film resists by lamination or as liquid photoresists by coating followed by drying. The coating methods used for printed circuit boards are dip coating, screen printing, roller coating, curtain coating or electrostatic spraying methods.
The vast majority of the printed circuit boards have conductor tracks on both sides which are produced by photochemical methods and are mutually connected by chemical plating metallization.
The term "printed circuit board" should be understood here to mean both the copper-clad insulating base material and also the already structured printed circuit board.
The vast majority of the photosensitive liquid photoresists are composed of radiation-sensitive components, binders, sensitizers, lacquer auxiliaries and also organic solvents. Before exposure, which is carried out, as a rule, in contact with film masters from both sides, the photoresist has to be dried so that the masters do not adhere to the photoresist layers during exposure.
The master is composed, in general, of a polyester film, but it may also be a glass tool.
The photosensitive liquid resist is dried after coating by heating the resist layer to approximately 70.degree. to 120.degree. C. In this process, on the one hand, the solvent evaporates; on the other hand, the resist layers and the reactive components, which contain, for example, monomers or diazonium salts, become soft again on heating since the drying temperatures are above the softening temperatures of the components in the vast majority of cases. As a consequence, these already dried layers are still sensitive to mechanical contact on heating. Specific resists, in particular the photosensitive solder resist masks, soften so easily that processing difficulties may even arise, for example in the exposure frame, in which temperatures of 35.degree. to 38.degree. C. readily occur and in which the boards are in contact with the film master in vacuo.
Single-sided application methods such as screen printing, electrostatic spraying methods and also curtain casting have established themselves as general coating techniques. Just as in pre-cleaning and during exposure, the boards are conveyed horizontally in these processes.
Thus, German Offenlegungsschrift 1,772,976 discloses a photomechanical method for producing printed circuit boards with plated-through holes, in which method the photosensitive layer, to which the surface pattern to be etched out later has first to be transferred photographically, is applied by means of an electrostatic resist-coating method.
Printed circuit boards which are resist-coated on both sides present the requirement to produce them in a manner such that no markings or adhesions to the surface of the board side resist-coated first appear on drying the other board side. Normally, the printed circuit boards are first resist-coated on one side and pre-dried and then, after turning over, are resist-coated on the second side in the same plant or in a second plant and finally, both sides are dried completely.
One drying possibility is to lay the boards in suitable racks and to dry them in a hot-air oven. To do this, the racks have, as a rule, to be loaded manually. On the plate racks there are board support points at which adhesion may also occur. Drying boards of different formats in said racks presents a particular difficulty; for this purpose, racks of different or variable size have to be employed.
Another drying possibility in the case of double-sided resist coating is presented by horizontal continuous ovens with V-shaped belts for conveying the boards. Only the edges of the resist-coated boards lie on the V-shaped belts. Since, however, the belt is heated up as it passes through the drier, the resist softens preferentially at these support points, and this may result in adhesion or in or in contamination of the belt. The V-shaped belts are furthermore unsuitable for drying flexible or bendable printed circuit boards such as are employed as internal layers for producing multilayer boards, since such printed circuit boards may sag or droop.
Instead of a V-shaped belt, it is also possible to employ suitably bent brackets. The contact area is then smaller but the contact pressure is higher. If boards of very different size are used, the spacing of the inclined belts or the angle has to be adjusted. This is important, in particular, when drying with IR radiators since the radiator/board distance has to be as constant as possible.
In a further method for drying double-sided resist-coated boards, driers with clip-chain board conveyance are employed. Here again the clips leave imprints behind since they heat up and enter the resist layer. In this case, resist particles may remain stuck to the clips which give rise to contamination in the drier. Furthermore, the matching to the plate size has to be carried out very carefully. To convey flexible boards, the chains have additionally to keep the boards taut.
Horizontal, continuously operated driers where the boards are conveyed on chains with laterally fitted pins are also employed for drying during printed circuit board production. In that case, the top side is heated with IR radiators and the bottom side is simultaneously cooled with an air jet. This is done, as a rule, to protect the components already mounted on the bottom side against overheating. This cooling air does not, however, prevent the entire chain and the conveyor pins from being heated up during the pass, and in this process the resist coating of the bottom side may be damaged.
The imprinting of the resist at the bottom side of the board and the contamination of the conveyor system can be avoided if the coating does not take place up to the edge of the board. This requires particular measures for limiting the resist coating, which measures have to be adapted each time the board format is altered. In order that the resist coating can be limited within a few millimeters of the edge zone, the boards furthermore have to be precisely oriented in one line and run in a straight line through the coating plant. With frequent format change such as is unavoidable in printed circuit board production, this demands a high expenditure on control.
Cooling belts for cooling molten resins or polymers, the melt being poured onto an endless metal belt and the endless belt being cooled from the bottom side by means of rollers, an air jet or liquids, including, for example, liquid nitrogen, are furthermore known. In this case, it is not, however, critical whether the solidified melt is imprinted by the belt surface or not. It would also be conceivable to employ a cooled conveyor belt for printed circuit board drying. However, the production of such endless metal belts is relatively expensive and it is difficult to keep the belt continuously in contact with a cooling surface. If that does not take place, the belt may heat up rapidly owing to its low heat capacity, and this again results in resist adherence.